1. Field of the Invention
The present invention relates to a noise filter used for a digital signal.
2. Description of the Related Art
Currently used widely in various fields are microprocessors which receive an input signal through a filter as a digital signal input according to a specification, environment, et cetera.
For instance, a patent document 1 (a laid-open Japanese patent application publication No. 2003-247453) comprises a filter, which is constituted by a capacitor and a resistor, equipped in a digital input terminal of an on-vehicle controller for inputting a signal through the filter.
And a patent document 2 (a laid-open Japanese patent application publication No. 11-41298 has disclosed a configuration to equip a noise filter of a digital circuit in a telecommunication circuit for removing a noise on a pulse overlapped with serial data input thereto.
When processing an analog signal by a digital circuit, the analog signal is converted into a digital signal by an A/D (analog to digital) conversion.
In a circuit using a processor, a rise/fall transition time (Tr/Tf) of an analog signal input to a microprocessor is not allowed to become steep in order to reduce noise.
Because of this, the duration of an input signal staying in the neighborhood of an input threshold value of the processor becomes long, during which time the input signal transmitted to the processor repeats changes between low and high levels.
Therefore, if the above described signal is used as a GPIO (general purpose input/output) input signal or other input signal for detecting an edge, a malfunction may result.
FIG. 1 illustrates the point.
FIG. 1A shows an input analog signal, while FIG. 1B shows the result of an A/D conversion of the analog signal shown by FIG. 1A by an A/D converter.
As shown by FIG. 1B, if a gradually changing signal is A/D converted, a transistor repeats on/off transitions near the threshold value thereof constituting the A/D converter, resulting in noise 2a and 2b occurring in the neighborhood of rise/fall edges of an internal detection signal 1.
A countermeasure to the above described noise uses an analog or digital noise filter.
Generally a digital noise filter determines a unique time for a signal to pass through the filter.
An occurrence of noise, however, depends on environmental factors such as a temperature of a deployed processor, a state of a printed circuit board, an operating speed, etc., and therefore it is not possible to determine the noise removal time uniquely.